Liposomes are vesicles comprising a lipid bilayer similar to that of the cell. In their interior there is an aqueous solution in which different molecules such as natural products, synthetic drugs or nucleic acid can be found. These vesicles are currently used as carriers and molecular delivery systems with many different applications. Typical applications include delivery of bioactive compounds in therapy or cosmetics.
Liposomes are constantly evolving, and some of the problems associated with their use in a first generation, as for example a short plasma half-life or a non-specific binding, have been partly solved (Torchilin V. “Recent advances with liposomes as pharmaceutical carriers”. Nature Rev. Drug Disc. 2005, vol. 4, pp. 145-160). With the aim of reducing the rapid elimination of liposomes in the circulation, they have been coated with soluble, inert and biocompatible polymers such as e.g. polyethylene glycol (PEG), which forms a protective layer that slows down the recognition by opsonins, thus increasing their bioavailability. As for the non-specific binding, which could cause the incorrect delivery of their contents, an increasing number of functionalized ligands on the surface of the liposomes have been tested, in an attempt to transform them into specific delivery systems. These targeting molecules can range from small molecules such as folate or biotin, to peptides or even antibodies.
Liposomes simultaneously functionalized with coating polymers and a guiding ligand, with the goal of trying to achieve a longer half-life and at the same time a more efficient and selective delivery of its content, have also been described. Thus, for example, liposomes have been described where one of membrane phospholipids (distearoylphosphatidylethanolamine, or DSPE) has been used as an anchor for the covalent binding of a chain of polyethylene glycol (PEG) which, in turn, has been used to covalently anchor a guiding ligand, in this case, a cyclic RGD peptide (Dubey P., et. al. “Liposomes modified with cyclic RGD peptide for tumor targeting”. J. Drug Targ. 2004, vol. 12, pp. 257-264). In the state of the art, other references disclosing liposomes with conjugates of the type phospholipid-PEG-[guiding ligand] are found. For example, liposomes with conjugates of the type DSPE-PEG-AG73 (Negishi Y., et. al. “Preparation and characterization of laminin-derived peptide AG73-coated liposomes as a selective gene delivery tool”. Biol. Pharm. Bull. 2010, vol. 33, pp. 1766-1769) have been described. AG73 is a peptide derived from the globular domain of the al chain of laminin, which is known to bind to syndecan-2, a receptor overexpressed in some cancers.
Although phospholipids have been used as anchoring points, other constituents of the liposomes have been explored, especially cholesterol. Thus, for example, references that describe conjugates of the type cholesterol-PEG-[guiding ligand] are found. A conjugate of cholesterol-PEG-Cetuximab has been described for the delivery of boron compounds on cancerous cells which overexpress the Cetuximab receptor, EGFR (Pan X., et. al. “Synthesis of cetuximab-immunoliposomes via a cholesterol-based membrane anchor for targeting of EGFR”. Bioconjugate Chem. 2007, vol. 18, pp. 101-108). Another example of the state of the art is the conjugate of the type cholesterol-PEG-RGD for the selective delivery of coding DNA in gene therapy on cells overexpressing integrin αvβ3 (Thomson B., et. al. “Neutral postgrafted colloidal particles for gene delivery”. Bioconj. Chem. 2005, vol. 16, pp. 608-614).
It should be noted that the covalent bonding between the cholesterol of the liposome membrane and the tandem PEG-[guiding ligand] in all of these references is achieved by an ester- or carbamate-type bond.
On the other hand, liposomes which have been functionalized with other groups have been described. In particular, there have been described functionalized liposomes with conjugates in which the cholesterol is bound to the PEG through a vinyl ether (called CVEP) (Boomer J., et. al. “Cytoplasmic delivery of liposomal contents mediated by an acid-labile cholesterol-vinyl ether-PEG conjugate” Bioconj. Chem. 2009, vol. 20, 47-69). The vinyl ether group is cleaved in an acidic medium, resulting in the loss of the PEG coating. These liposomes additionally contain a second conjugate of DSPE-PEG-folate that directs them to cells overexpressing the folate receptor.
Although some technical problems associated with the use of liposomes have been partially resolved, there is still a need for conjugates that confer better physico-chemical, and pharmacological properties to the carrying liposomes.